Lesson 5/Learning Event 1
Two kinds of thermostats in common use are the bellows and pellet type. Both types operate on the same
principle; the difference is in the unit that opens and closes the valve. The bellows type consists of a flexible
metal bellows attached to a valve. The sealed bellows is filled with a liquid such as ether. When the liquid is
cold, the bellows is contracted so the valve is closed. When heated, the liquid turns to a vapor and expands the
bellows, opening the valve. This thermostat is sensitive to outside pressure, so it does not work very well if the
cooling system is pressurized.